第四届全国中学生物理竞赛预赛第一试有这样一个题目: 如图1所示,P为一块均匀的半圆形薄电阻合金片。先将它按图甲的方式接在电极A、B之间,测得它的电阻为R。然后再将它按图乙的方式接在电极C、D之间,这时P的电阻值为多少? 参考资料采用了一种等效法解答。笔者以为还可用另外两种等效法求解。等效法1:设想给甲乙两图所示的两极间补上另一完全相同的半圆形薄电阻合金片。如图2所示,丙图中两个半圆金属薄片 The first trial of the fourth national junior high school physics competition preliminaries has such a problem: As shown in Figure 1, P is a uniform semi-circular thin resistive alloy sheet. First, it is connected between electrodes A and B in the way of Fig. A, and its resistance is measured as R. Then connect it to the electrode C, D according to Figure B. What is the resistance value of P? The reference material uses an equivalent method to solve. The author thinks that it can also be solved by the other two equivalent methods. Equivalent method 1: It is assumed that another two identical semicircular thin resistance alloy sheets are added between the two poles shown in FIG. As shown in Figure 2, two semi-circular foils in C